1. Field of the Invention
The present invention relates generally to communication systems, and more particularly, to an apparatus for time-aligning signals on a reverse link in code division multiple access (CDMA) communication systems.
2. Description of the Related Art
In a CDMA system, each user transmits in the same RF band. To avoid mutual interference, different code sequences are used to identify and distinguish users. These user code sequences are generally divided into orthogonal code sequences and non-orthogonal code sequences.
Typically the use of non-orthogonal codes as a user identifying code involves assigning different seed numbers to users and then processing PN (Pseudo Noise) sequences with the seed numbers. Due to the non-orthogonality of such codes, user signals suffer from interference from other user signals, making signal separation to be less than ideal. Orthogonal functions improve the bandwidth efficiency of a CDMA system. While there are many different code sequences that can be used to generate an orthogonal set of functions, the Walsh and Hadamard sequences are among the most popular for CDMA. Walsh codes, for example, are characterized by no interference among users in the same cell only if time alignment is ensured, thus enabling user signals to be separated from one another. Without time alignment, if a multipath causes a delay of one of the user""s codes by one chip, for example, then the delayed code may no longer be orthogonal to the other non-delayed codes in the code set. Therefore, time alignment is essential when using Walsh or Hadamard codes for DS-SS multiple access. In practice, the IS-95 CDMA system uses a pilot channel and a sync channel to time align (i.e., synchronize) the forward link and to ensure that the link is coherent.
CDMA mobile communication systems employ both orthogonal and non-orthogonal code sequences for forward and reverse links. The forward link is typically orthogonally coded because a base station simultaneously generates signals to a number of user terminals. Currently, Walsh codes are used only on the forward link. Non-orthogonal code sequences are typically used on the reverse link. The base station receives non-time aligned signals from the terminals, which makes it difficult to use orthogonal codes for user identification.
A time alignment method for synchronous reception of signals in a base station of a CDMA mobile communication system is disclosed in U.S. Pat. No. 5,446,727, to Bruckert et al. (xe2x80x9cthe ""727 patentxe2x80x9d). The ""727 patent discloses that time alignment of reverse link transmission may be achieved by designating a time position in the strongest signal path as that of a multipath signal received from a terminal or computing an average arrival time from arrival times of signals from different paths as the time position of a multipath signal, and then periodically transmitting a time alignment signal to a terminal,
However, the method disclosed in the ""727 patent relies on measuring the strength of an occupied channel and may be inaccurate by not being properly time aligned. Also, a time alignment signal should be sent on the occupied channel for control of time alignment.
Periodic transmission of a time alignment signal from a base station to a terminal in a conventional CDMA mobile communication system is not effective for certain situations for terminals which are relatively slow and thus show little variation in channel status. Another associated problem is that limitations exist in managing a possible sudden change of channel status in terminals. A need therefore exists for a method and apparatus for time alignment of a reverse link transmission during data transmission.
An object of the present invention is, therefore, to provide a method and apparatus for time alignment of a reverse link transmission in a CDMA mobile communication system.
Another object of the present invention is to provide a method and an apparatus for time alignment of a reverse link transmission in a base station of a CDMA mobile communication system.
A further object of the present invention is to provide a method and an apparatus for time alignment of a reverse link transmission in a CDMA mobile communication system, in which a base station transmits a time alignment signal on a dedicated control channel (DCCH) and a mobile terminal time-aligns (i.e., in communication) occupied reverse channels based on the time alignment signal.
Still another object of the present invention is to provide a method and an apparatus for time alignment of a reverse link transmission in a CDMA mobile communication system, in which time alignment is executed on a predetermined reverse channel among several reverse channels in current use.
To achieve the above objects, there is provided a method and apparatus for time alignment on a reverse link in a CDMA mobile communication system; the system comprising a base station having a reference sync channel with a round trip delay (RTD) time. The method comprises the step of calculating an RTD error value as the difference between a reference sync channel RTD and a sync channel RTD, and outputting a time alignment control signal on a forward control channel if the computed RTD error exceeds a predetermined value during a data communication. A terminal receives the time alignment control signal on a forward control channel and time aligns a reverse traffic channel based on the received time alignment control signal.